The invention relates to a tunable laser oscillator comprising a laser for producing an output signal, and a control unit for applying a first tuning signal to a first control input of the laser for tuning the laser to a desired frequency.
A laser oscillator of this type is known from U.S. Pat. No. 4,914,666. Such laser oscillators are used, for example, in transmitters or receivers for coherent optical transmission systems.
To transport a baseband signal via a glass fibre in coherent optical transmission systems, a light signal coming from a transmitting laser can be amplitude, frequency or phase modulated by the baseband signal before the light signal is fed to the glass fibre.
To demodulate the light signal at a receiver with the aid of current electronic components, it is necessary to convert the light signal, which has a very high frequency (for example, 10.sup.14 Hz), to a much lower intermediate frequency of, for example, 10.sup.9 Hz. For this purpose, the received light signal is combined in the receiver with a local laser-generated light signal with the aid of a photodiode. This combination provides an intermediate frequency signal which has a frequency equal to the difference frequency between the frequency of the received light signal and that of the locally generated light signal.
To simultaneously transport more than a single light signal via a glass fibre, lasers which are tunable over a large frequency range (for example, 500 GHz) are used in both the transmitter and the receiver. As a result, more transmitters and receivers can communicate via the same glass fibre without causing interference with one another.
The frequency of the light signal generated by a tunable laser depends, for example, on the value of one or more electrical signals which are applied to one or more control inputs of the laser. In prior-art laser oscillators, a laser which has two control inputs is used. The same tuning signal, generated by the control unit, is applied to these control inputs through two resistors. The relation between a desired frequency and the associated tuning signal is determined in the control unit. This relation may be determined by means of a single measurement of the frequency of the light generated by the laser as a function of the tuning signal. The control signal may be, for example, a current flowing through an active part of the laser, or one which determines, for example, the temperature or another ambient condition of the laser.
During the tuning operation in prior-art laser oscillators, the frequency of the light signal generated by the laser may change in a jumpy fashion which causes hysteresis, i.e., when the tuning signal is increased slowly, a frequency jump occurs at a value different from when the tuning signal is reduced. This is undesired because the relation between the tuning signal and the wavelength of the light signal generated by the laser is ambiguously fixed, i.e., two different frequencies may belong to a single value of the tuning signal.